A measurement of the dependence of the direct energy gap of GaAs on hydrostatic pressure is reported. Pressures up to 180 kbar were applied with a diamond anvil cell and measured using the ruby luminescence technique. At 180 kbar the material experiences a transition to a phase which is opaque in the visible region of the electromagnetic spectrum. The energy gap E0(eV)=1.45+1.26×10-2P-3.77×10-5P2 (P is the pressure in kbar) exhibits a strong nonlinear dependence on P. The dependence of E0 on the lattice parameter is more nearly linear but still sublinear in the relative change of lattice constant. A theoretical study of this nonlinearity is carried out using the method of the pseudopotential. We find that the observed sublinearity of E0 on the lattice parameter can be accounted for by supposing that the core radius varies quadratically with the relative change of this quantity. Finally, our results are compared with those of the empirical dielectric theory. © 1975 The American Physical Society.